A bistatic radar is a radar with transmit and receive antennas separated by a considerable distance with respect to target range. In recent years, these bistatic and multistatic radars are gaining more and more attention because they can provide low-cost, ECCM capabilities against stealth targets.
Due to the geometry of bistatic or multistatic radars, e.g. such as described in U.S. Patent Application No. 20060202885, “Operational Bistatic Radar System Synchronization,” P. Chen. Sep. 14, 2006, incorporated herein by reference, which describes a direct line-of-sight (LOS) connection employing QPSK bit synchronization, the synchronization of time and frequency at the transmitters and receivers is a crucial problem for coherent signal processing and range measurement. The coherent integration of signals from remote nodes for cohere-on-transmit and cohere-on-receive require very stringent and hard phase locking and synchronization. Typically, the remote nodes must be synchronized to less than a few percent of a carrier frequency. Also, the clock at each node must be phase locked not to cause drift of the summed signals during the coherent integration time. These become even more difficult when the nodes are on a moving platform as in the case of synthetic aperture radar (SAR).
When the nodes are separated by a line-of-sight distance, direct measurements using hard-wired cables (fiber-optic or RF) or free-space (free-space optics or free-space RF wave) communications are used to achieve synchronization on the order of ns.
When the nodes are separated beyond the line-of-sight, satellites are required. Rubidium or quartz clocks using four or more GPS space vehicles, e.g. such as described in U.S. Pat. No. 6,995,705, “System And Method For Doppler Track Correlation For Debris Tracking”, Bradford et al., 2006, incorporated herein by reference, are most commonly used to achieve synchronization of remote nodes to tens of ns. However, in adverse environments with multipath interference or moving platforms, performances become significantly degraded. Furthermore, timing depends on propagation delays which depend on sensor locations. To extract the desired timing portion from the measurements, precise location is also necessary. Other representative bistatic or multistatic radar systems that employ direct LOS include: U.S. Patent Application No. 20050128135, “Remote Phase Synchronization Using A Low-Bandwidth Timing Referencer”, Hester et al., Jun. 16, 2005; U.S. Pat. No. 4,021,804, “Synchronized, Coherent Timing System For Coherent-On-Receive Radar System”, Eric A. Dounce, May 3, 1977; U.S. Pat. No. 5,361,277, “Method And Apparatus For Clock Distribution And For Distribuuted Clock Synchronization”, Wayne D. Grover, Nov. 1, 1994; U.S. Pat. No. 6,297,765, “Bistatic Passive Radar System With Improved Ranging”, Lawrence M. Frazier, Oct. 2, 2001; U.S. Pat. No. 7,589,665, “Multistatic Method And Device For Radar Measuring A Close Distance”, P. Heide et al., Sep. 15, 2009; and U.S. Pat. No. 5,818,371, “Coherent Synchronization And Processing Of Pulse Groups”, C. Lu et al., Oct. 6, 1998, all of which are incorporated herein by reference.
Time transfer is a method for transferring a reference clock from one point to another over a long distance. Due to the recent advances in global positioning system, navigation, etc., time transfer has become an important element. Various methods of time transfer have been developed over many years including one-way transfer, two-way transfer, and common view transfer. However, these methods lack precision mainly due to the incomplete cancellation of propagation delays.